Bulk packaging of leafy, stringy and other low bulk density vegetables with a uniform package product weight is a difficult and time consuming process. Historically, such vegetables after blanching have been packaged by hand into cans. Hand filling of cans requires the operator to fill, weigh and then add or remove product to obtain product weights within the weight tolerance range set for the product. This is a slow and operator intensive process and is generally limited to no more than 5 cans per minute per operator.
Alternatives to hand packing leafy vegetables using complex mechanical devices such as described in U.S. Pat. No. 4,019,547 have been developed but they are used to fill metal cans which are disappearing from commerce due to their weight and expense. There is a strong desire among producers and consumers to replace the heavy cans with light weight polymer films.
Wax cartons with wax overwrap also have been used for packaging vegetables such as spinach in 10 ounce packages. Those packages also are labor-intensive and costly.
Leafy vegetables such as spinach and lettuce have been packaged fresh into polymer bags, but are limited in the weight of each bag, the shelf life of the packaged product and the special film and atmosphere requirements of respiring vegetables. While this may be acceptable for individual consumers for the preparation of fresh salads, it does not meet the needs of institutional kitchens or large volume suppliers for bulk packaging of leafy vegetables for cooking or reheating and serving.
Therefore, it has been a goal to develop a packaging film and packaging method for the packaging of high solid vegetable slurries for frozen storage, transport and institutional use to replace cans and wax cartons. Replacing cans and wax cartons for vegetables will reduce the transportation weight of the vegetables, and permit the frozen storage of the vegetables which will prove the nutritional quality and taste of the final cooked vegetable product. The use of high solids slurries will improve the product packaged density and improve the uniformity of the filling process. However, the packaging of high solids slurries produces sealing surfaces which are potentially contaminated with solid vegetable. This solid contamination has been found to inhibit the sealing of the bags with traditional heat sealing methods usually employed on VFF&S packaging machines.
It has been discovered that ultrasonic sealing methods can seal through dry solid contamination, using traditional polyolefin packaging films. However, when the solids are combined with water into slurries, the water captured with the solids in the ultrasonic sealing area produce holes in the seal areas which lead to product leakage and package failure. This wet product sealing difficulty can be overcome using laminations, such as produced with oriented PET films combined with coextruded or single layer polyethylene sealants. Without being limited by a particular theory, it is believed that during ultrasonic sealing of the wet sealing surfaces, that as the polyethylene film of the bag wall melts from the action of the ultrasonic energy, the water on the film surface and/or contained in the vegetable slurry is converted to water vapor or steam by the ultrasonic energy and the steam released during sealing perforates the molten polyethylene film layers creating pin holes, voids, weak seal areas and leaks in the seal area, all of which render the seal unusable for the application. The high temperature layer of PET in the lamination is believed to support the molten polyethylene sealing layer from being sufficiently distorted as to form pin holes, voids and leaks in the seal area.
Even though the laminations can be successfully sealed to form packages of high solids content vegetable slurries, they are considerably more expensive than coextruded packaging films and due to the considerably poorer low temperature ductility of the PET film, suffer significant package breakage in cold storage and transport.
While the use of higher melting polypropylene to replace lower melting polyethylene core layers in multilayer coextrusions would be sufficient to reduce or eliminate the steam perforation of the seal area, polypropylene homopolymer layers have poor low temperature ductility at freezer temperatures and are known to give considerable package breakage in cold storage and transport. Copolymer and terpolymer polypropylene layers will reduce the melting point sufficiently to give layer perforation without a significant improvement in cold storage performance. Consequently, the use of polypropylene or copolymer and terpolymer propylene layers in the current film structures has not proven adequate when the product is to be frozen for storage and transport.
All prior known techniques are either too expensive for long term economic performance or limit the cold storage and shipping of the frozen vegetable products.